What are laser ‘smoke rings’?

Lasers are usually thought of as a beam of light that travels in one straight line until it hits a mirror. Over the course of its journey, a laser gradually expands due to the wave nature of light but this basic rule doesn’t apply to high-intensity light.

According to the University of Maryland physicists, ‘powerful laser beams, given the right conditions, will act as their own lenses and “self-focus” into a tighter even more intense beam’. These are the types of laser the scientists have been studying and they have discovered something rather interesting.

The self-focussed laser pulses are also generating ‘violent swirls of optical energy that strongly resemble smoke rings’. Described as ‘spatiotemporal optical vortices’ (STOVs) or more simply ‘donut-shaped’, the light energy flows around the inside of the ring and loops back around the outside.

Howard Milchberg, professor and senior author of the research paper told Subcon Laser, ‘STOVs appear to be a new ‘knob’ with which to manipulate light. Just like high order transverse modes of laser beams can have phase jumps in space across the beam face, we’ve discovered that light pulses can have phase jumps in time. The STOVs are the locations of the phase jumps. They may have possible uses as bits in optical communications. They could also provide enhanced time resolution for time resolved studies including microscopy.’

If ‘spatiotemporal optical vortices’ or ‘STOVs’ don’t normally crop up in your day to day chit chat this might help as a very basic guide:

Imagine having a long string pulled taut and then sending spinning smoke rings down the line. The string is the laser travelling in a single direction and the smoke rings are the STOVs that spin around the laser.

Image: Howard Milchberg

Explaining years of anomalies

Milchberg said, ‘Lasers have been researched for decades, but it turns out that STOVs were under our noses the whole time.’ He goes on to say, ‘this phenomenon underlies so much that’s been done in our field for the past 30-some years.’

While conventional spatial optical vortices (made with normal light and appearing as a ring of light with a dark centre) have been studied before and have applications in high-resolution microscopy, this is a first for lasers.

The paper’s lead author, Nihal Jhajj said, ‘the smoke ring vortices we discovered may have even broader applications than previously known optical vortices, because they are time dynamic, meaning that they move along with the beam instead of remaining stationary. This means that the rings may be useful for manipulating particles moving near the speed of light.’

Realising how fundamental STOVs are

Milchberg told us, ‘We saw STOVs in our calculations and our measurements and were at first amazed at how robust they were and then confused about what they meant and what role they were playing. But then we realized how fundamental and important they are to the propagation of high intensity laser beams–they act as power directors within the beam.’

There is plenty more research to be done but the potential held in this phenomenon is very exciting. We look forward to seeing what the Maryland Physicists get up to next.